Helicopter

Background

Helicopters are classified as rotary wing aircraft, and their rotary wing
is commonly referred to as the
main rotor
or simply the rotor. Unlike the more common fixed wing aircraft such as a
sport biplane or an airliner, the helicopter is capable of direct vertical
take-off and landing; it can also hover in a fixed position. These
features render it ideal for use where space is limited or where the
ability to hover over a precise area is necessary. Currently, helicopters
are used to dust crops, apply
pesticide,
access remote areas for environmental work, deliver supplies to workers
on remote maritime oil rigs, take photographs, film movies, rescue people
trapped in inaccessible spots, transport accident victims, and put out
fires. Moreover, they have numerous intelligence and military
applications.

Numerous individuals have contributed to the conception and development of
the helicopter. The idea appears to have been bionic in origin, meaning
that it derived from an attempt to adapt a natural phenomena—in
this case, the whirling, bifurcated fruit of the maple tree—to a
mechanical design. Early efforts to imitate maple pods produced the
whirligig, a children's toy popular in China as well as in medieval
Europe. During the fifteenth century, Leonardo da Vinci, the renowned
Italian painter, sculptor, architect, and engineer, sketched a flying
machine that may have been based on the whirligig. The next surviving
sketch of a helicopter dates from the early nineteenth century, when
British scientist Sir George Cayley drew a twin-rotor aircraft in his
notebook. During the early twentieth century, Frenchman Paul Cornu managed
to lift himself off the ground for a few seconds in an early helicopter.
However, Cornu was constrained by the same problems that would continue to
plague all early designers for several decades: no one had yet devised an
engine that could generate enough vertical thrust to lift both the
helicopter and any significant load (including passengers) off the ground.

Igor Sikorsky, a Russian engineer, built his first helicopter in 1909.
When neither this prototype nor its 1910 successor succeeded, Sikorsky
decided that he could not build a helicopter without more sophisticated
materials and money, so he transferred his attention to aircraft. During
World War I, Hungarian engineer Theodore von Karman constructed a
helicopter that, when tethered, was able to hover for extended periods.
Several years later, Spaniard Juan de la Cierva developed a machine he
called an
autogiro
in response to the tendency of conventional airplanes to lose engine
power and crash while landing. If he could design an aircraft in which
lift and thrust (forward speed) were separate functions, Cierva
speculated, he could circumvent this problem. The autogiro he subsequently
invented incorporated features of both the helicopter and the airplane,
although it resembled the latter more. The autogiro had a rotor that
functioned something like a windmill. Once set in motion by taxiing on the
ground, the rotor could generate supplemental lift; however, the autogiro
was powered primarily by a conventional airplane engine. To avoid landing
problems, the engine could be disconnected and the autogiro brought gently
to rest by the rotor, which would gradually cease spinning as the machine
reached the ground. Popular during the 1920s and 1930s, autogiros ceased
to be produced after the refinement of the conventional helicopter.

The helicopter was eventually perfected by Igor Sikorsky. Advances in
aerodynamic theory and building materials had been made since
Sikorsky's initial endeavor, and, in 1939, he lifted off the ground
in his first operational helicopter. Two years later, an improved design
enabled him to remain aloft for an hour and a half, setting a world record
for sustained helicopter flight.

The helicopter was put to military use almost immediately after its
introduction. While it was not utilized extensively during World War II,
the jungle terrain of both Korea and Vietnam prompted the
helicopter's widespread use during both of those wars, and
technological refinements made it a valuable tool during the Persian Gulf
War as well. In recent years, however, private industry has probably
accounted for the greatest increase in helicopter use, as many companies
have begun to transport their executives via helicopter. In addition,
helicopter shuttle services have proliferated, particularly along the
urban corridor of the American Northeast. Still, among civilians the
helicopter remains best known for its medical, rescue, and relief uses.

Design

A helicopter's power comes from either a piston engine or a gas
turbine (recently, the latter has predominated), which moves the rotor
shaft, causing the rotor to turn. While a standard plane generates thrust
by pushing air behind its wing as it moves forward, the
helicopter's rotor achieves lift by pushing the air beneath it
downward as it spins. Lift is proportional to the change in the
air's momentum (its mass times its velocity): the greater the
momentum, the greater the lift.

Helicopter rotor systems consist of between two and six blades attached to
a central hub. Usually long and narrow, the blades turn relatively slowly,
because this minimizes the amount of power necessary to achieve and
maintain lift, and also because it makes controlling the vehicle easier.
While light-weight, general-purpose helicopters often have a two-bladed
main rotor, heavier craft may use a four-blade design or two separate main
rotors to accommodate heavy loads.

To steer a helicopter, the pilot must adjust the pitch of the blades,
which can be set three ways. In the
collective
system, the pitch of all the blades attached to the rotor is identical;
in the
cyclic
system, the pitch of each blade is designed to fluctuate as the rotor
revolves, and the third system uses a combination of the first two. To
move the helicopter in any direction, the pilot moves the lever that
adjusts collective pitch and/or the stick that adjusts cyclic pitch; it
may also be necessary to increase or reduce speed.

Unlike airplanes, which are designed to minimize bulk and protuberances
that would weigh the craft down and impede airflow around it, helicopters
have unavoidably high drag. Thus, designers have not utilized the sort of
retractable landing gear familiar to people who have watched planes taking
off or landing—the aerodynamic gains of such a system would be
proportionally insignificant for a helicopter. In general, helicopter
landing gear is much simpler than that of airplanes. Whereas the latter
require long runways on which to reduce forward velocity, helicopters have
to reduce only vertical lift, which they can do by hovering prior to
landing. Thus, they don't even require shock absorbers: their
landing gear usually comprises only wheels or skids, or both.

One problem associated with helicopter rotor blades occurs because airflow
along the length of each blade differs widely. This means that lift and
drag fluctuate for each blade throughout the rotational cycle, thereby
exerting an unsteadying influence upon the helicopter. A related problem
occurs because, as the helicopter moves forward, the lift beneath the
blades that enter the airstream first is high, but that beneath the blades
on the opposite side of the rotor is low. The net effect of these problems
is to destabilize the helicopter. Typically, the means of compensating for
these unpredictable variations in lift and drag is to manufacture flexible
blades connected to the rotor by a hinge. This design allows each blade to
shift up or down, adjusting to changes in lift and drag.

Torque, another problem associated with the physics of a rotating wing,
causes the helicopter fuselage (cabin) to rotate in the opposite direction
from the rotor, especially when the helicopter is moving at low speeds or
hovering. To offset this reaction, many helicopters
use a tail rotor, an exposed blade or ducted fan mounted on the end of
the tail boom typically seen on these craft. Another means of
counteracting torque entails installing two rotors, attached to the same
engine but rotating in opposite directions, while a third, more
space-efficient design features twin rotors that are enmeshed, something
like an egg beater. Additional alternatives have been researched, and at
least one NOTAR (no tail rotor) design has been introduced.

Raw Materials

The airframe, or fundamental structure, of a helicopter can be made of
either metal or organic composite materials, or some combination of the
two. Higher performance requirements will incline the designer to favor
composites with higher strength-to-weight ratio, often epoxy (a resin)
reinforced with glass, aramid (a strong, flexible nylon fiber), or carbon
fiber. Typically, a composite component consists of many layers of
fiber-impregnated resins, bonded to form a smooth panel. Tubular and sheet
metal substructures are usually made of aluminum, though
stainless steel
or titanium are sometimes used in areas subject to higher stress or heat.
To facilitate bending during the manufacturing process, the structural
tubing is often filled with molten sodium silicate. A helicopter's
rotary wing blades are usually made of fiber-reinforced resin, which may
be adhesively bonded with an external sheet metal layer to protect edges.
The helicopter's windscreen and windows are formed of polycarbonate
sheeting.

The Manufacturing
Process

Igor Sikorsky pilots his craft, the VS-300, close to the ground in
this 1943 demonstration.

In 1939, a Russian emigre to the United States tested what was to become
a prominent prototype for later helicopters. Already a prosperous
aircraft manufacturer in his native land, Igor Sikorsky fled the 1917
revolution, drawn to the United States by stories of Thomas Edison and
Henry Ford.

Sikorsky soon became a successful aircraft manufacturer in his adopted
homeland. But his dream was vertical take-off, rotary wing flight. He
experimented for more than twenty years and finally, in 1939, flew his
first flight in a craft dubbed the VS
300.
Tethered to the ground with long ropes, his craft flew no higher than
50 feet off the ground on its first several flights. Even then, there
were problems: the craft flew up, down, and sideways, but not forward.
However, helicopter technology developed so rapidly that some were
actually put into use by U.S. troops during World War II.

The helicopter contributed directly to at least one revolutionary
production technology. As helicopters grew larger and more powerful, the
precision calculations needed for engineering the blades, which had
exacting requirements, increased exponentially. In 1947, John C. Parsons
of Traverse City, Michigan, began looking for ways to speed the
engineering of blades produced by his company. Parsons contacted the
International Business Machine Corp. and asked to try one of their new
main frame office computers. By 1951, Parsons was experimenting with
having the computer's calculations actually guide the machine
tool. His ideas were ultimately developed into the
computer-numerical-control (CNC) machine tool industry that has
revolutionized modern production methods.

William S. Pretzer

Airframe: Preparing the tubing

1 Each individual tubular part is cut by a tube cutting machine that can
be quickly set to produce different, precise lengths and specified batch
quantities. Tubing requiring angular bends is shaped to the proper angle
in a bending machine that utilizes interchangeable tools for different
diameters and sizes. For other than minor bends, tubes are filled with
molten sodium silicate that hardens and eliminates kinking by causing
the tube to bend as a solid bar. The so-called
water glass
is then removed by placing thebent tube in boiling water, which melts
the inner material. Tubing that must be curved to match fuselage
contours is fitted over a stretch forming machine, which stretches the
metal to a precisely contoured shape. Next, the tubular details are
delivered to the machine shop where they are held in clamps so that
their ends can be machined to the
required angle and shape. The tubes are then deburred (a process in
which any ridges or fins that remain after preliminary machining are
ground off) and inspected for cracks.

2 Gussets (reinforcing plates or brackets) and other reinforcing details
of metal are machined from plate, angle, or extruded profile stock by
routing, shearing, blanking, or sawing. Some critical or complex details
may be forged or investment cast. The latter process entails injecting
wax or an alloy with a low melting point into a mold or die. When the
template has been formed, it is dipped in molten metal as many times as
necessary to achieve the thickness desired. When the part has dried, it
is heated so that the wax or alloy will melt and can be poured out.
Heated to a higher temperature to purify it and placed in a mold box
where it is supported by sand, the mold is then ready to shape molten
metal into reinforcement parts. After removal and cooling, these parts
are then finish-machined by standard methods before being deburred once
again.

3 The tubes are chemically cleaned, fitted into a subassembly fixture,
and MIG (metal-arc inert gas) welded. In this process, a small electrode
wire is fed through a welding torch, and an inert, shielding gas
(usually argon or helium) is passed through a nozzle around it; the
tubes are joined by the melting of the wire. After welding, the
subassembly is stress relieved—heated to a low temperature so
that the metal can recover any elasticity it has lost during the shaping
process. Finally, the welds are inspected for flaws.

Forming sheet metal details

4 Sheet metal, which makes up other parts of the airframe, is first cut
into blanks (pieces cut to predetermined size in preparation for
subsequent work) by abrasive water-jet, blanking dies, or routing.
Aluminum blanks are heat-treated to anneal them (give them a uniform,
strain-free structure that will increase their malleability). The blanks
are then refrigerated until they are placed in dies where they will be
pressed into the proper shape. After forming, the sheet metal details
are aged to full strength and trimmed by routing to final shape and
size.

5 Sheet metal parts are cleaned before being assembled by riveting or
adhesive bonding. Aluminum parts and welded subassemblies may be
anodized (treated to thicken the protective oxide film on the surface of
the aluminum), which increases corrosion resistance. All metal parts are
chemically cleaned and primer-painted, and most receive finish
paint
by spraying with epoxy or other durable coating.

Making the cores of composite components

6 Cores, the central parts of the composite components, are made of
Nomex (a brand of aramid produced by Du Pont) or aluminum
"honeycomb," which is cut to size by bandsaw or
reciprocating knife. If necessary, the cores then have their edges
trimmed and beveled by a machine tool similar to a pizza cutter or meat
slicing blade. The material with which each component is built up from
its cores (each component may use multiple cores) is called
pre-preg ply.
The plies are layers of oriented fibers, usually epoxy or polyimide,
that have been impregnated with resin. Following written instructions
from the designers, workers create highly contoured skin panels by
setting individual plies on bond mold tools and sandwiching cores
between additional plies as directed.

7 Completed
layups,
as the layers of prepreg affixed to the mold are called, are then
transported to an autoclave for curing. An autoclave is a machine that
laminates plastics by exposing them to pressurized steam, and
"curing" is the hardening that occurs as the resin layers
"cook" in the autoclave.

8 Visible trim lines are molded into the panels by scribe lines present
in the bond mold tools. Excess material around the edges is then removed
by bandsawing. Large panels may be trimmed by an abrasive water-jet
manipulated by a robot. After inspection, trimmed panels and other
composite details are cleaned and painted by normal spray methods.
Surfaces must be well sealed by paint to prevent metal corrosion or
water absorption.

Making the fuselage

9 Canopies or windscreens and passenger compartment windows are
generally made of polycarbonate sheet. Front panels

Most of the crucial components in a helicopter are made of metal
and are formed using the usual metal-forming processes: shearing,
blanking, forging, cutting, routing, and investment casting. The
polycarbonate windscreen and windows are made by laying the sheet
over a mold, heating it, and forming it with air pressure in a
process called 'freeblowing," in which no tool ever
touches the part.

subject to bird strike or other impact may be laminated of two sheets
for greater thickness. All such parts are made by placing an oversized
blank on a fixture, heating it, and then forming it to the required
curvature by use of air pressure in a freeblowing process. In this
method, no tool surface touches the optical surfaces to cause defects.

Installing the engine, transmission,
and rotors

10 Modern helicopter engines are turbine rather than piston type and are
purchased from an engine supplier. The helicopter manufacturer may
purchase or produce the transmission assembly, which transfers power to
the rotor assembly. Transmission cases are made of aluminum or magnesium
alloy.

11 As with the above, the main and tail rotor assemblies are machined
from specially selected high-strength metals but are produced by typical
machine shop methods. The rotor blades themselves are machined from
composite layup shapes. Main rotor blades may have a sheet metal layer
adhesively bonded to protect the leading edges.

Systems and controls

12 Wiring harnesses are produced by laying out the required wires on
special boards that serve as templates to define the length and path to
connectors. Looms, or knitted protective covers, are placed on the wire
bundles, and the purchased connectors are soldered in place by hand.
Hydraulic tubing is either hand-cut to length and hand-formed
by craftsmen, or measured, formned, and cut by tube-bending machines.
Ends are flared, and tubes are inspected for dimensional accuracy and to
ensure that no cracks are present. Hydraulic pumps and actuators,
instrumentation, and electrical devices are typically purchased to
specification rather than produced by the helicopter manufacturer.

Final assembly

13 Finished and inspected detail airframe parts, including sheet metal,
tubular, and machined and welded items, are delivered to subassembly
jigs (fixtures that clamp parts being assembled). Central parts are
located in each jig, and associated details are either bolted in place
or, where rivets are to be used, match-drilled using pneumatically
powered drills to drill and ream each rivet hole. For aerodynamic
smoothness on sheet metal or composite skin panels, holes are
countersunk so that the heads of flat-headed screws won't
protrude. All holes are deburred and rivets applied. A sealant is often
applied in each rivet hole as the rivet is inserted. For some
situations, semi-automated machines may be used for moving from one hole
location to the next, drilling, reaming, sealing, and installing the
rivets under operator control.

14 After each subassembly is accepted by an inspector, it typically
moves to another jig to be further combined with other small
subassemblies and details such as brackets. Inspected "top
level" subassemblies are then delivered to final assembly jigs,
where the overall helicopter structure is integrated.

Upon completion of the structure, the propulsion components are added,
and wiring and hydraulics are installed and tested. Canopy, windows,
doors, instruments, and interior elements are then added to complete
the vehicle. Finish-painting and trimming are completed at appropriate
points during this process.

15 After all systems are inspected in final form, along with physical
assemblies and appearance aspects, the complete documentation of
materials, processes, inspection, and rework effort for each vehicle is
checked and filed for reference. The helicopter propulsion system is
tested, and the aircraft is flight-tested.

Quality Control

Once tubular components have been formed, they are inspected for cracks.
To find defects, workers treat the tubes with a fluorescent liquid
penetrant that seeps into cracks and other surface flaws. After wiping off
the excess fluid, they dust the coated tube with a fine powder that
interacts with the penetrant to render defects visible. After the tubular
components have been welded, they are inspected using X-ray and/or
fluorescent penetrant methods to discover flaws. Upon completion, the
contours of sheet metal details are checked against form templates and
hand-worked as required to fit. After they have been autoclaved and
trimmed, composite panels are ultrasonically inspected to identify any
possible breaks in laminations or gas-filled voids that could lead to
structural failure. Prior to installation, both the engine and the
transmission subassemblies are carefully inspected, and special test
equipment, custom-designed for each application, is used to examine the
wiring systems. All of the other components are also tested before
assembly, and the completed aircraft is flight-tested in addition to
receiving an overall inspection.

The Future

Manufacturing processes and techniques will continue to change in response
to the need to reduce costs and the introduction of new materials.
Automation may further improve quality (and lower labor costs). Computers
will become more important in improving designs, implementing design
changes, and reducing the amount of paperwork created, used, and stored
for each helicopter built. Furthermore, the use of robots to wind
filament, wrap tape, and place fiber will permit fuselage structures to be
made of fewer, more integrated pieces. In terms of materials, advanced,
high-strength thermoplastic resins promise greater impact resistance and
repairability than current thennosets such as epoxy and polyimide.
Metallic composites such as aluminum reinforced with boron fiber, or
magnesium reinforced with silicon carbide particles, also promise higher
strength-to-weight ratios for critical components
such as transmission cases while retaining the heat resistance advantage
of metal over organic materials.

I always find it frustrating how authors of aviation related articles do not understand the theory of flight. A rotary wing (any wing for that matter) generates lift NOT by forcing air downward from itself, but by producing a low pressure area ABOVE the wing. The curvature on the upper part of a wing makes the air travelling over it move faster than the air below the wing - and since the upper air molecules are spread apart more (moving faster), a slight vacuum is created. Since nature abhors a vacuum, the wing tends to move upward to fill the void (or low pressure area) as a result. This is why it is called lift - not 'push' or upward thrust. Lift. Simple.

well, david, I have to say you seem to be a little rude. It's true, what you explain, but it is also true what the article says!
Both of the explanations are sides of the same coin. Or is it false that the air goes downward after pasing through the wing? It is, moreover, the principle of action-reaction (newton) there couldn't exist such a simple lift, if nothing would be pushed downward. simple :)

David, your right, that is the principle behind flight. I too, find it frustrating that people often fail to explain the theory of flight correctly. even though the air is forced down and the rotor creates a fan-like machine, to say that a rotor creates upward thrust and that it is, in essence, a jet engine forcing the craft upward is entirely incorrect. The rest of the article was very interesting and as far as I know, accurate.